Historical seismicity and implications for diffuse plate convergence in the northeast Indian Ocean
Historical seismicity and implications for diffuse plate convergence in the northeast Indian Ocean
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历史地震活动及其对东北印度洋扩散板块汇聚的影响
DOI：
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发表时间：
1989
期刊：
影响因子：
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通讯作者：
D. Wiens
中科院分区：
文献类型：
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作者：
D. Petroy;D. Wiens
历史上的大地震对从中印度海岭延伸至苏门答腊海沟的缓慢、扩散的板块边界的地理范围和运动学提供了限制。 1963 年之前地震的断层参数是通过网格搜索导出的，以求观测到的和合成的物体波形、表面波频谱幅度以及极性和极化约束之间的最佳拟合，并使用自举重采样方法确定位置不确定性。 1928 年 3 月 9 日的九十东岭 (M 7.7) 事件显示，沿与九十东岭方向大致重合的平面存在左旋走滑断层。 1980 年在 1928 年地震发生地点发生的两起小事件显示出震源机制与 1928 年地震确定的震源机制几乎相同，并且显然重新激活了同一断层。 1928 年的波形也可以通过 ENE 冲击断层面的逆冲断层解来拟合，但各种证据，包括最近在同一地点发生的走滑地震，都表明了走滑解。这些地震位于山脊的西部陡坡，并不代表该大厦以东的九十东断裂带重新激活，该断裂带是新生代早期的主要板块边界。 1949年1月23日（震级6.75）和1955年3月22日（震级7.0）沃顿盆地地震虽然相距仅350公里，却表现出不同的走滑和逆冲断层机制。所有地震都表现出区域一致的水平压应力模式，从沃顿盆地的西北-东南向到中印度盆地的南北向变化。压缩轴的方向垂直于与岩石圈弯曲相关的线性重力异常，表明自折叠开始以来应力方向几乎没有变化。沿扩散边界的地震变形包括与南北向特征相关的左旋走滑断层，如九十东岭和86°E断裂带，以及断裂带之间区域的逆冲或逆断层。假设发生变换运动，根据地震矩总和计算出的沿九十东岭北部的近似滑动率为 2.8 毫米/年。该值比之前的估计小一个数量级，表明当前相对板块运动最多只有一小部分（~7毫米/年）沿着山脊发生。地震活动的地理分布表明，九十东岭以东的沃顿盆地几乎与西部的中印度盆地一样地震活跃。这一结果与沃顿盆地观测到的长波长重力异常一起表明，印度-澳大利亚板块边界沿着数千公里长的扩散变形区域而不是沿着离散的转换带与苏门答腊海沟相交。
Large historical earthquakes provide constraints on the geographical extent and kinematics of the slow, diffuse plate boundary extending from the Central Indian Ridge to the Sumatra Trench. The fault parameters of pre-1963 earthquakes are derived using a grid search for the best fit between observed and synthetic body waveforms, surface wave spectral amplitudes, and polarity and polarization constraints, and location uncertainties are determined using a bootstrap resampling method. The March 9, 1928 Ninetyeast Ridge (M 7.7) event shows left lateral strike-slip faulting along a plane approximately coincident with the orientation of the Ninetyeast Ridge. Two small events which occurred at the location of the 1928 event in 1980 show focal mechanisms nearly identical to that determined for the 1928 earthquake and apparently reactivated the same fault. The 1928 waveforms can also be fit by a thrust faulting solution with ENE striking fault planes, but a variety of evidence, including the recent strike-slip earthquakes at the same location, suggest the strike-slip solution. These earthquakes are located along the western scarp of the ridge and do not represent reactivation of the Ninetyeast Fracture Zone to the east of the edifice, which was a major plate boundary in the early Cenozoic. The January 23, 1949 (M 6.75) and March 22, 1955 (M 7.0) Wharton Basin earthquakes, although only 350 km apart, show different strike-slip and thrust faulting mechanisms. All earthquakes show a regionally consistent pattern of horizontal compressional stress, which changes from NW-SE in the Wharton Basin to N-S in the Central Indian Basin. The compressional axes are oriented perpendicular to linear gravity anomalies associated with lithospheric flexure, indicating little change in stress orientation since the onset of folding. Seismic deformation along the diffuse boundary consists of sinistral strike-slip faulting associated with N-S features such as the Ninetyeast Ridge and the 86°E fracture zone, and thrust or reverse faulting in regions between fracture zones. Assuming transform motion, the approximate slip rate along the northern Ninetyeast Ridge calculated from the summed moments of earthquakes is 2.8 mm/yr. This value is an order of magnitude less than previous estimates, and suggests that at most only a small fraction of the current relative plate motion (∼7 mm/yr) occurs along the ridge. The geographic distribution of seismicity suggests that the Wharton Basin east of the Ninetyeast Ridge is nearly as seismically active as the Central Indian Basin to the west. This result, along with observed long wavelength gravity anomalies in the Wharton Basin, indicates that the India-Australia plate boundary intersects the Sumatra Trench along a diffusely deforming region several thousand kilometers long, rather than along a discrete transform zone.